Women who cook, roast, dehydrate or blend their babies’ placentas into smoothies are engaging in a practice that flirts with cannibalism, according to the authors of a new review on the human “placentophagy” phenomenon.

Advocates — including private placenta preparers, often working in their own kitchens and who charge upwards of $400 to rinse, dry and grind placentas into powdered capsules — claim consuming the placenta after childbirth helps improve mood and energy, soothe “hormonal crashes” and increase milk flow.

However, the new review finds no scientific evidence of any clinical benefit of placentophagy but risks of harm, including the possible transmission of serious bacterial infections to newborns.

“Most mammals eat the placenta after birth, but we can only guess why they do so,” Dr. Alex Farr, of the Medical University of Vienna and one of the review’s authors, said in a statement. One theory is that animals did it to hide any sign of childbirth from predators. Regardless, Farr and his co-authors suggest any benefit may be “species specific” and not translatable to humans.

According to the review, published online ahead of print in the American Journal of Obstetrics and Gynecology, some women are eating slices of placenta raw, directly after childbirth; others are deep-freezing the organs for later consumption. “Placental material might also be mixed with fruits or juices to create smoothies that mask the unpleasant taste,” the team writes, “or might be used as a meat substitute for recipes such as lasagne or pasta.”

The most popular preparation, however, is the encapsulation method, whereby frozen placenta is rinsed, “stripped of membranes, cleaned, sliced, steamed and dehydrated” at 46 to 71 degrees Celsius and then pulverized and placed into capsules to be taken daily by women in the weeks following birth.

Studies in rats suggest ingested placenta can act like an opioid, increasing the rodent’s pain tolerance. The placenta also contains prostaglandins, which help the uterus contract.

However, Farr and colleagues argue none of the placental nutrients or hormones are likely retained in sufficient quantities after processing to be of any benefit to a human mother.

The only published randomized, double blind, placebo-controlled study in women looked at the effect of encapsulated placenta on women’s postpartum iron levels. No statistically significant differences in iron levels were found between women in the placenta group versus those taking a placebo made of beef.

More worrisome, the U.S. Centers for Disease Control and Prevention last year linked infected placenta pills with possible transmission of group B streptococcus in a baby.

An Oregon newborn developed strep B shortly after birth and was treated with an antibiotic for 11 days. Five days after it was sent home, the baby became irritable and was taken to emergency. A blood test showed the baby had strep B again. The baby’s mother had paid a company to pick up and encapsulate her placenta; she had been swallowing two capsules, three times a day. The dried placenta inside the capsules tested positive for strep and the bacteria found in the pills were identical to those found in the cerebrospinal fluid of the baby.

Although the CDC couldn’t rule out that another household member may have infected the baby, “the report is the first solid evidence that contaminated placenta capsules can be a source of infection,” Farr and his co-authors from Weill Cornell Medicine at Cornell University in New York write.

Other studies have raised concerns about cadmium, a heavy metal found in low but detectable amounts in placenta pills. “Women who ingest multiple pills may risk cumulative dosing,” the team reports, adding that it may explain why women who eat placenta frequently report headaches.

In an email interview, Farr said that while some might find his reference to cannibalism offensive, “from a medical perspective, the placenta carries the fetal genome and therefore it belongs to the baby. It is up to every individual whether he or she considers eating human, genetically-different tissue as cannibalism or not.”

Middle to upper class white women with relatively high incomes and education are among the most frequent consumers of placenta, he said. “This might be due to the fact that these women have access and frequently follow (pseudo) fashionable trends and often prefer esoteric and alternative medicine.”

One concern is that bioactive hormones like estradiol, an estrogen, may survive the encapsulation process. While those hormonal effects might, theoretically, lower the risk of post-partum depression, he said, they could also increase the risk of stroke or heart attack from blood clots.

“At the moment it is completely unknown whether the intake of these additional hormones can cause any harm,” Farr said.

He and his co-authors say doctors have no professional responsibility to offer placentophagy to pregnant women and should discourage any woman who asks from eating her placenta, which provides oxygen and nutrients to the developing fetus.

“There had just been a theoretical risk of harm, so we considered it patients’ choice to eat the placenta or not,” he said. “Since the latest CDC warning, there is now however evidence for a significant risk of harm.”

Still, the number of Google searches for placenta encapsulation continues to rise, fuelled by celebrities like Kim Kardashian.

The Society of Obstetricians and Gynaecologists of Canada is developing its own position statement.

“It’s probably going to be a very similar statement — that we cannot support placental encapsulation consumption at this time,” said Dr. Julie van Schalkwyk, head of obstetrics and gynecology at B.C. Women’s Hospital. For one thing, no standards exist for people processing placentas, increasing the risk of contamination.

It’s not clear how many women giving birth in hospitals are asking to take their placentas home, or even which hospitals allow it. In the U.S., some hospitals strictly refuse, considering the organ a potential biohazard.

At B.C Women’s, new mothers have to sign a waiver releasing the hospital of responsibility for any harm that might occur from handling their placentas.

“I don’t have a firm number as to how many women are doing what, and in what sort of state they’re consuming their placentas,” van Schalkwyk said.

However, “it’s a practice we know is happening.”

The human placenta is a “substantial thing,” says University of Saskatchewan professor of obstetrics and gynecology Dr. Roger Pierson. The organ typically weighs 500 to 600 grams, measures 23 to 25 cm in diameter and is vascular “in the extreme.”

“The animals that eat their placentas are not humans, not traditionally,” Pierson said. “People can’t be serious. My goodness, what are (they) thinking?”

He said the practice is more similar to autophagy, or “self-eating”, than cannibalism. “In this variant, you’ve created this as a by-product of a different type of biological process, you deliver it and you eat it.” Outside of a bit of protein, “I can’t imagine any of the hormonal components that are sequestered in that tissue making it into the vascular system of a person who eats it,” he said.